Increased glycolysis is a biochemical hallmark of neoplastic cells (Warburg, 1956). Overexpression of glucose transporters in the cell-membranes of cancer cells results in increased glucose uptake (Kim and Dang, 2006). Furthermore, increased activity of glycolytic enzymes, such as hexokinase, phosphorylates glucose to prohibit its exit from the cell (Parry and Pedersen, 1983). This results in increased accumulation and consumption of glucose by cancer cells. Two families of glucose transporters have been reported in humans: GLUT [Solute Carrier family 2 (facilitated glucose transporter), gene name SLC2A] and SGLT [Solute carrier family 5 (sodium/glucose cotransporter); gene name: SLC5A]. While members of the GLUT family of transporters rely on existing concentration gradients to transport glucose from one side of the membrane to the other, members of the SGLT family transport sugars inside the cell against a concentration gradient by active transport. To date, 14 members of the GLUT family have been identified (GLUT1-14) (Thorens and Mueckler). Although there is significant sequence homology among the GLUTs, they display differential expression in tissues and have varying affinities for glucose and related sugars. GLUT1; GLUT2, and GLUTS have been shown to be overexpressed in cancer. GLUT3 is primarily expressed on neurons, while GLUT4 and GLUT12 are primarily expressed in cardiac and skeletal muscle and their presence in the cell membrane is regulated by insulin. Six members of the SGLT family have been identified (SGLT1-6). Of these, SGLT-1 and SGLT-2 are implicated in transport of sugars. Expression of SGLT-1 in normal tissues is restricted to the small intestine and renal proximal tubes, while SGLT-2 is expressed on the apical membranes of cells in the renal convoluted proximal tubules. SGLT-1 has high affinity and low capacity for glucose transport, while SGLT-2 has lower affinity and higher capacity. (Thorens and Mueckler) Increased expression of both SGLT-1 and SGLT-2 have been reported in primary and metastatic cancers (Helmke et al., 2004, Ishikawa et al., 2001, Weihua et al., 2008)
The increased accumulation of glucose in neoplastic cells has been exploited for development of diagnostic and therapeutic agents. Radiolabeled carbohydrates have been developed for nuclear imaging of cancer. The most successful of these, 2-18F-fluoro-2-deoxyglucose (FDG) is currently the gold standard for positron emission tomography (PET). Other sugar-based PET agents have also been developed, including 18F-Fluoroacetyl-D-glucosamine (Fujiwara et al., 1990), 68Ga-2-amino-2-deoxyglucose-based 3-hydroxy-4pyridonato complexes (Green et al., 2005), 68Ga-tripodal trissalicylaldimine complexes functionalized with xylose, glucose or galactose (Gottschaldt, 2009) 11C-methyl-glucoside (US2010/0008856, Bormons et al, 2003). Radiolabeled sugars have also been developed for SPECT imaging including 99mTc-Glucosamine-dipicolylamine, US2006/0051291) 99mTc-tricarbonyl-N-(2′-Hydroxybenzyl)-2-amino-2-deoxy-D-glucose, (US2006/0051291) 99mTc Ehtylenedicysteine-glucosamine (US 2007/0248537), 111In-DOTA-glucosamine, (US2006/0142207A1), 99mTc-DTPA-deoxyglucose (Chen et al., 2007)
Solid phase approach is a robust, efficient and reproducible method of synthesis of library of compounds. It was successfully applied for the synthesis of peptides, nucleotides, and glycopeptides. In solid phase, synthesized compounds are temporary attached to the insoluble polymer resins, allowing them to be readily separated from coupling agents and by-products during elongation process. After completion of the synthesis, final compound is released from the polymer support with high yield and purity. Application of solid phase approach for the development of imaging or therapeutic agents is still limited. SPPS was used for the synthesis peptidyl imaging agents (U.S. Patent Application Publication No. 2008/0089842). Amino-functionalized chelator was immobilized on the resin and coupled to the C-terminus and/or backbone of the peptide. After cleaving from the resin, amino-chelator-peptide was labeled with lanthanide metals.
Target-specific radiopharmaceuticals consist of a tissue targeting biomolecule attached to a radionuclide. Commonly used cyclotron-produced radionuclides (i.e. 11C, 13N, 15O and 18F) may be covalently linked to targeting molecules. Alternatively, complexes consisting of a targeting ligand, bifunctional chelating agent and a radionuclide, can be used for nuclear imaging and therapy. In this case, the radionuclide, most commonly a radiometal, is stably coordinated by the bifunctional chelating agent (BFCA). Common BFCAs for radiometallic chelation include DTPA, hydrazinonicotinamide (HYNIC), mercaptoacetyltriglycine (MAG3), tetraaza compounds (i.e. 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid [DOTA] and macrocyclic derivatives) and ethylenedicysteine (EC). Each BFCA possesses various combinations of electron-donating atoms for metal chelation. For diagnostic radiopharmaceuticals, γ-emitting radiometals such as 68Ga and 64Cu are commonly used for PET, and 99mTc, 111In and 67Ga are commonly used for SPECT. Additionally, α-emitting (225Ac and 213Bi) and β-emitting (90Y, 177Lu, and 186/188Re) radiometals can be used for radionuclide therapy.
Diagnostic imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) provide anatomical information about disease sites. While these modalities are commonly-used for monitoring changes in tumor size, they cannot assess functional changes occurring within cells or tumors. Functional imaging modalities use radiotracers to image, map and measure biological attributes of disease, such as metabolism, proliferation and surface receptor expression. As a result, functional imaging techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) have been experiencing explosive growth due to advances in molecular imaging technology. New molecular imaging targets for diagnosis and therapy have been developed to visualize disease states and pathological processes without surgical exploration of the body. In particular, targeted radiopharmaceuticals offer promising capabilities for the non-invasive assessment of the pathophysiology of diseases. Schillaci, O. & Simonetti, G., Cancer Biother. Radiopharm. 19: 1-10 (2004); Paulino, et al, Semin Nucl. Med. 33: 238-43 (2003).
In contrast to traditional radiotherapy methods, radionuclide therapy utilizes active targeting for improved tumor specificity and localization of radiation in tumor cells, thereby minimizing the effects on normal tissue. This approach uses high-affinity cell binding ligands to target radioactivity to tissues expressing a particular receptor. Radiolabeled monoclonal antibodies were among the first targeted therapies developed and have shown moderate clinical success, particularly for the treatment of Non-Hodgkin's lymphoma. However, the large size, limited diffusion and slow blood clearance of the tumor-seeking antibodies have limited their clinical utility. To address these concerns, radiolabeled peptides and low/intermediate molecular weight biologically-active proteins have been employed into the design of novel radionuclide therapies (Pnwar et al., 2005). The smaller size of these molecules confers desirable pharmacokinetic properties that are favorable for therapy, such as higher target-to-background ratios and faster blood clearance.